1. Field of the Invention
The present invention relates to a technique for transferring digital data using, for example, a satellite broadcast, a CATV (cable television) or a public or other network and, in particular, to a technique which can ensure data identity and security upon data transfer.
2. Description of the Prior Art
Owing to development of the data communication technique and the multimedia technique, utilization or application of digital data (hereinafter also referred to as "information") has been largely expanded and, following this, there have been various data transfer methods, for example, using the satellite broadcast, the CATV, the Internet and the Intranet. Under the circumstances, when transferring highly valuable or secret information to a user from a provider, particularly using the public or other network which is not under control of the provider, the reliable security for preventing an unjust or unfair reception of the information by a third party is essential, in addition to ensuring the identity of the contents of the information.
For example, when transferring chargeable information relating to movie, electronic publication, home medical care, education or the like from a provider to members in the membership communication system, change in content of the information or unfair reception and utilization of the information by a third party, other than the member, should be prevented. A cryptographic technique is effective as a preventive measure thereof. Particularly, when putting much weight on ensuring the identity of the contents of the information, a stream cryptographic technique is quite effective.
In the stream cryptographic technique, the same random number bit sequences are generated at a data transmitter device (encrypting or encoding side) and a data receiver device (decrypting or decoding side), an exclusive OR operation is performed per bit between the random number bit sequence and the information so as to derive the cipher information (encrypted information) at the data transmitter device, and an exclusive OR operation is performed per bit between the same random number bit sequence and the cipher information at the data receiver device so as to decode the cipher information. Since the processing is performed per bit of the information, an error caused during data transfer is not expanded upon decrypting the cipher information so that this technique is suitable for transferring the large-amount information at high rate and with high accuracy. The cryptographic techniques including the stream cryptographic technique are described in "Introduction to Information Theory" (by EiJi Okamoto, Kyoritsu Publishing, 1993).
As described above, when reproducing the original information from the cipher information at the data receiver device, the exclusive OR operation is performed per bit relative to the cipher information using the same random number bit sequence as was used at the data transmitter device. To obtain the same random number bit sequences at the data transmitter and receiver devices is called synchronization. There are available an external synchronization method and a self-synchronization method. In the external synchronization method, data transmitter and receiver devices produce random number bit sequences independently of each other and send data for synchronization to each other. On the other hand, in the self-synchronization method, a data receiver device autonomously produces a random number bit sequence based on received cipher information and thus synchronization is automatically achieved after receipt of a given fixed length of the cipher information.
However, in either of the foregoing synchronization methods, random number generating means are required both at the data transmitter device and at the data receiver device. Accordingly, in the communication system where a plurality of data receiver devices exist relative to one data transmitter device, the installation cost increases largely. Further, since the process for synchronization is essential, it is not suitable for the high-speed processing. Particularly, it takes much time to achieve synchronization in the self-synchronization method and thus it has been very difficult to deal with information, such as dynamic images (animation), requiring rapid processing. Further, in the stream cryptographic technique, it has been a serious problem to ensure difficulty in interpreting a random number bit sequence which is a key for encryption.